This invention relates to the identification of telephone customer loops as loaded loops and nonloaded loops and, more particularly, to an arrangement for electronically performing such customer loop identification. For this application, a loaded loop is defined as a customer 2-wire metallic loop into which lumped inductive loads have been connected to reduce the effects of distributed loop capacitance in accordance with well-known telephone loop design practices.
Until recently, local telephone switching offices generally switched telephone messages as analog signals on 2-wire metallic switching paths. Such analog local switching offices are nearly transparent to the transmission of telephone messages and introduce minimal signal loss (typically less than 0.5 db) across the switching network. Accordingly, present transmission design practices only include an allowance for this negligible cross office loss.
However, with the introduction of local digital switching, cross office loss may no longer be negligible. The use of a digital switch, which is inherently a 4-wire device, with existing analog transmission facilities requires a 2-wire to 4-wire to 2-wire conversion. This conversion is performed by hybrid circuits with their attendant stability problems. That is, a signal injected into the forward transmission path can pass across the hybrid and return to the starting point through the reverse transmission path. The resulting return signals can cause reverberations which make telephone messages sound distorted or give the voice signals a hollow sound. If the return signals are sufficiently high, the circuit goes into oscillation which is referred to as singing. The stability of the hybrid circuits can be assured by increasing the loss around the 4-wire path through the digital switch. However, any increase in cross office loss beyond that encountered in an analog switching office is in conflict with existing transmission design practices and could result in degradation of transmitted signals.
A well-known solution to the dilemma of providing low cross office loss while preventing singing is to provide improved balance networks for the hybrids. Improved balance networks provide better impedance balance at the hybrids to reduce the return signals. It is further known that additional singing margin can be gained by utilizing two different balance networks, one for loaded loops and one for nonloaded loops. This separate treatment for loaded loops and nonloaded loops is referred to as "loop segregation".
Local telephone switching offices maintain office records which identify each of the customers' loops and indicate whether those loops are loaded or nonloaded. By referring to the office records, loop segregation can be performed by connecting the appropriate balance network in accordance with the records. Unfortunately, office records are often inaccurate and out of date. Additional error may be introduced by inaccurate use of the office records and, as the office records are frequently updated, frequent changes are required. Furthermore, a loop which is properly identified in the office records but improperly designed may have the quality of service degraded by applying the balance network indicated by the office records. These problems lead to administrative headaches as well as introducing a high probability of error in the treatment applied to the customer loops served by a local switching system.